Cardiac resynchronization therapy devices operate by either delivering pacing stimulus to both ventricles or to one ventricle with the desired result of a more or less simultaneous mechanical contraction and ejection of blood from the ventricles. Ideally, each pacing pulse stimulus delivered to a ventricle evokes a response from the ventricle. Delivering electrical stimuli that causes the ventricle to respond is commonly referred to as capturing a ventricle.
For a variety of reasons, cardiac pacing systems may not effectively capture a ventricle. For example, a patient experiencing atrial fibrillation (AF) may have an implantable cardioverter-defibrillator (ICD) that shows a very high percentage (e.g., greater than 90%) of delivered biventricular (BiV) paces compared to sensing of intrinsic activations. However, BiV pacing may be ineffective if the paces are delivered to myocardial tissue in a refractory state (i.e. subnormal excitability of myocardial tissue following excitation), resulting in pseudo-fusion. Pseudo-fusion involves electrical activation of ventricles almost entirely through intrinsic electrical activity with minimal or no contribution from pacing of the ventricle(s). Pseudo-fusion can occur during AF, because AF results in irregular conduction of atrial impulses to the ventricles. In turn, irregular ventricular activation rate increases the chance of inconsistent capture of the ventricles. Typically, delivery of substantially consistent BiV pacing during AF is achieved by overdriving the ventricular intrinsic rate. However, overdriving ventricular rate can lead to unfavorable cardiac mechanics and can exacerbate heart failure. Therefore, it is desirable to increase the percent of effective BiV pacing during AF without significantly increasing the ventricular rate.